White, opaque, spodumene glass-ceramic articles with tunable color and methods for making the same

ABSTRACT

Crystallizable glasses, glass-ceramics, IXable glass-ceramics, and IX glass-ceramics are disclosed. The glass-ceramics exhibit β-spodumene ss as the predominant crystalline phase. These glasses and glass-ceramics, in mole %, include: 62-75 SiO 2 ; 10.5-18 Al 2 O 3 ; 5-14 Li 2 O; 2-12 B 2 O 3 ; and 0.4-2 Fe 2 O 3 . Additionally, these glasses and glass-ceramics can exhibit the following criteria: 
     
       
         
           
             a 
              
             
                 
             
              
             ratio 
              
             
               
                 
                   : 
                 
                  
                 
                     
                 
                 [ 
                 
                   
                     
                       
                         Li 
                         2 
                       
                        
                       O 
                     
                     + 
                     
                       
                         Na 
                         2 
                       
                        
                       O 
                     
                     + 
                     
                       
                         K 
                         2 
                       
                        
                       O 
                     
                     + 
                     MgO 
                     + 
                     ZnO 
                   
                   _ 
                 
                 ] 
               
                
               
                   
               
               [ 
               
                 
                   Al 
                   2 
                 
                  
                 
                   O 
                   3 
                 
               
               ] 
             
           
         
       
         
         
           
             between 0.8 to 1.5.
 
The glass-ceramics also exhibit colors at an observer angle of 10° and a CIE illuminant F02 determined with specular reflectance of a* between −0.5 and 0.5, b* between −2.5 and +2, and L* between 90 and 93.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 62/018,921 filed on Jun. 30, 2014the content of which is relied upon and incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present disclosure relates to crystallizable glasses (precursorglasses crystallizable to glass-ceramics), glass-ceramics, ionexchangeable (“IXable”) glass-ceramics, and/or ion exchanged (“IX”)glass-ceramics; processes for making the same; processes for tuning thecolor of the same; and articles comprising the same. In particular, thepresent disclosure relates to crystallizable glasses (precursor glassesformulated to be crystallizable to white, opaque glass-ceramicsincluding β-spodumene solid solution as a predominant crystallinephase); white, opaque, β-spodumene glass-ceramics; IXable, white,opaque, β-spodumene glass-ceramics; and/or IX, white, opaque,β-spodumene glass-ceramics; processes for making the same; processes fortuning the color of the same; and articles comprising the same.

BACKGROUND

In the past decade, as electronic devices such as notebook computers,personal digital assistants (PDAs), portable navigation device (PNDs),media players, mobile phones, portable inventory devices (PIDs) . . .etc. (frequently referred to as “portable computing devices”) haveconverged while at the same time becoming small, light, and functionallymore powerful. One factor contributing to the development andavailability of such smaller devices is an ability to increasecomputational density and operating speed by ever decreasing electroniccomponent sizes. However, the trend to smaller, lighter, andfunctionally more powerful electronic devices presents a continuingchallenge regarding design of some components of the portable computingdevices.

Components associated with portable computing devices encounteringparticular design challenges include the enclosure or housing used tohouse the various internal/electronic components. This design challengegenerally arises from two conflicting design goals—the desirability ofmaking the enclosure or housing lighter and thinner, and thedesirability of making the enclosure or housing stronger and more rigid.Lighter enclosures or housings, typically thin plastic structures withfew fasteners, tend to be more flexible while having a tendency tobuckle and bow as opposed to stronger and more rigid enclosure orhousings, typically thicker plastic structures with more fastenershaving more weight. Unfortunately, the increased weight of the stronger,more rigid plastic structures might lead to user dissatisfaction, whilethe bowing and buckling of the lighter structures might damage theinternal/electronic components of the portable computing devices andalmost certainly lead to user dissatisfaction.

Among known classes of materials are glass-ceramics that are used widelyin various other applications. For example, glass-ceramics are usedwidely in kitchens as cooktops, cookware, and eating utensils, such asbowls, dinner plates, and the like. Transparent glass-ceramics are usedin the production of oven and/or furnace windows, optical elements,mirror substrates, and the like. Glass-ceramics are typically made bycrystallizing crystallizable glasses at specified temperatures forspecified periods of time to nucleate and grow crystalline phases in aglass matrix. Two glass-ceramics based on the SiO₂—Al₂O₃—Li₂O glasssystem comprise those having either β-quartz solid solution (“β-quartzss” or “β-quartz”) as the predominant crystalline phase or β-spodumenesolid solution (“β-spodumene ss” or “β-spodumene”) as the predominantcrystalline phase.

As stated, in view of the foregoing problems with existing enclosures orhousings, there exists a need for materials such as crystallizableglasses (precursor glasses formulated to be crystallizable toglass-ceramics) and/or β-spodumene glass-ceramics and/or IXable,β-spodumene glass-ceramics and/or IX, β-spodumene glass-ceramics thatprovide, potentially in a more cost effective manner, improvedenclosures or housings for portable computing devices. Also, thereexists a need for such materials that provide improved whiteness levelsand/or opaque colors while addressing in an aesthetically pleasingmanner the design challenges of creating light, strong, and rigidenclosures or housings.

SUMMARY

Some aspects of embodiments and/or embodiments (“aspects and/orembodiments”) of this disclosure relate to crystallizable glassesformulated to be crystallizable to white, opaque, β-spodumeneglass-ceramics having β-spodumene as the predominant crystalline phase.Such crystallizable glasses, in mole percent (mole %), include: SiO₂ ina range from about 62 to about 75; Al₂O₃ in a range from about 10 toabout 18; Li₂O in a range from about 5 to about 14; B₂O₃ in a range fromabout 2 to about 12; and Fe₂O₃ in a range from about 0.4 to about 2,while in alternative aspects, in mole %, including: SiO₂ in a range fromabout 62 to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂Oin a range from about 5 to about 14; B₂O₃ in a range from about 2 toabout 12; MgO in a range from 0 to about 8; ZnO in a range from 0 toabout 4; TiO₂ in a range from about 2 to about 5; Na₂O in a range from 0to about 5; K₂O in a range from 0 to about 4; SnO₂ in a range from about0.05 to about 0.5; and Fe₂O₃ in a range from about 0.4 to about 2, whilein other alternative aspects, in mole %, including: SiO₂ in a range fromabout 62 to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂Oin a range from about 5 to about 14; B₂O₃ in a range from about 2 toabout 12; and a metal oxide selected from group consisting of CoO,Cr₂O₃, Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinationsthereof in a range from about 0.01 to about 2, while in still furtheralternative aspects, in mole %, SiO₂ in a range from about 62 to about75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a range fromabout 5 to about 14; B₂O₃ in a range from about 2 to about 12; MgO in arange from 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ in arange from about 2 to about 5; Na₂O in a range from 0 to about 5; K₂O ina range from 0 to about 4; SnO₂ in a range from about 0.05 to about 0.5;and a metal oxide selected from group consisting of CoO, Cr₂O₃, Cu₂O,Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof in arange from about 0.01 to about 2.

Additionally, in some embodiments the crystallizable glasses can exhibitthe following compositional criteria:

the ratio of:

-   -   the mole sum total of [Li₂O+Na₂O+K₂O+MgO+ZnO] to the moles of        [Al₂O₃]

can be in a range from about 0.8 to about 1.5.

Some other aspects and/or embodiments of this disclosure relate towhite, opaque, β-spodumene glass-ceramics having β-spodumene as thepredominant crystalline phase. Such white, opaque, β-spodumeneglass-ceramics, in mole %, include: SiO₂ in a range from about 62 toabout 75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a rangefrom about 5 to about 14; B₂O₃ in a range from about 2 to about 12; andFe₂O₃ in a range from about 0.4 to about 2, while in alternativeaspects, in mole % including: SiO₂ in a range from about 62 to about 75;Al₂O₃ in a range from about 10 to about 18; Li₂O in a range from about 5to about 14; B₂O₃ in a range from about 2 to about 12; MgO in a rangefrom 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ in a rangefrom about 2 to about 5; Na₂O in a range from 0 to about 5; K₂O in arange from 0 to about 4; SnO₂ in a range from about 0.05 to about 0.5;and Fe₂O₃ in a range from about 0.4 to about 2, while in furtheralternative aspects, in mole %, including: SiO₂ in a range from about 62to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a rangefrom about 5 to about 14; B₂O₃ in a range from about 2 to about 12; anda metal oxide selected from group consisting of CoO, Cr₂O₃, Cu₂O, Sb₂O₃,In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof in a range fromabout 0.01 to about 2, while in still further alternative aspects, inmole %, SiO₂ in a range from about 62 to about 75; Al₂O₃ in a range fromabout 10 to about 18; Li₂O in a range from about 5 to about 14; B₂O₃ ina range from about 2 to about 12; MgO in a range from 0 to about 8; ZnOin a range from 0 to about 4; TiO₂ in a range from about 2 to about 5;Na₂O in a range from 0 to about 5; K₂O in a range from 0 to about 4;SnO₂ in a range from about 0.05 to about 0.5; and a metal oxide selectedfrom group consisting of CoO, Cr₂O₃, Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO,V₂O₃, Ta₂O₅, and combinations thereof in a range from about 0.01 toabout 2.

Additionally, in some embodiments, such white, opaque, β-spodumeneglass-ceramics can exhibit the following compositional criteria:

the ratio of:

${the}\mspace{14mu} {mole}\mspace{14mu} {sum}{\mspace{11mu} \;}{total}\mspace{14mu} {{of}\mspace{14mu}\lbrack \underset{\_}{{{Li}_{2}O} + {{Na}_{2}O} + {K_{2}O} + {MgO} + {ZnO}} \rbrack}\mspace{14mu} {to}\mspace{14mu} {the}\mspace{14mu} {moles}\mspace{14mu} {{of}\mspace{14mu}\lbrack {{Al}_{2}O_{3}} \rbrack}$

can be in a range from about 0.8 to about 1.5.

Still other aspects and/or embodiments of this disclosure relate tomethods for forming crystallizable glasses formulated to becrystallizable to white, opaque, β-spodumene glass-ceramics and methodsfor forming white, opaque, β-spodumene glass-ceramics having aβ-spodumene as the predominant crystalline phase. In aspects, somemethods included melting a mixture of raw materials formulated toproduce upon melting crystallizable glasses, in mole %, including: SiO₂in a range from about 62 to about 75; Al₂O₃ in a range from about 10 toabout 18; Li₂O in a range from about 5 to about 14; B₂O₃ in a range fromabout 2 to about 12; and Fe₂O₃ in a range from about 0.4 to about 2,while in alternative aspects, in mole %, including: SiO₂ in a range fromabout 62 to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂Oin a range from about 5 to about 14; B₂O₃ in a range from about 2 toabout 12; MgO in a range from 0 to about 8; ZnO in a range from 0 toabout 4; TiO₂ in a range from about 2 to about 5; Na₂O in a range from 0to about 5; K₂O in a range from 0 to about 4; SnO₂ in a range from about0.05 to about 0.5; and Fe₂O₃ in a range from about 0.4 to about 2, whilein further alternative aspects, in mole %, including: SiO₂ in a rangefrom about 62 to about 75; Al₂O₃ in a range from about 10 to about 18;Li₂O in a range from about 5 to about 14; B₂O₃ in a range from about 2to about 12; and a metal oxide selected from group consisting of CoO,Cr₂O₃, Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinationsthereof in a range from about 0.01 to about 2, while in still furtheralternative aspects, in mole %, including: SiO₂ in a range from about 62to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a rangefrom about 5 to about 14; B₂O₃ in a range from about 2 to about 12; MgOin a range from 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ ina range from about 2 to about 5; Na₂O in a range from 0 to about 5; K₂Oin a range from 0 to about 4; SnO₂ in a range from about 0.05 to about0.5; and a metal oxide selected from group consisting of CoO, Cr₂O₃,Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof ina range from about 0.01 to about 2.

In additional aspects, such mixture of raw materials is formulated toproduce upon melting crystallizable glasses exhibiting the followingcompositional criteria:

the ratio of:

${the}\mspace{14mu} {mole}\mspace{14mu} {sum}{\mspace{11mu} \;}{total}\mspace{14mu} {{of}\mspace{14mu}\lbrack \underset{\_}{{{Li}_{2}O} + {{Na}_{2}O} + {K_{2}O} + {MgO} + {ZnO}} \rbrack}\mspace{14mu} {to}\mspace{14mu} {the}\mspace{14mu} {moles}\mspace{14mu} {{of}\mspace{14mu}\lbrack {{Al}_{2}O_{3}} \rbrack}$

can be in a range from about 0.8 to about 1.5.

In further aspects, some other methods include methods for formingglass-ceramics having a β-spodumene as the predominant crystalline phaseby transforming crystallizable glasses. Such other methods including (i)heating a crystallizable glass formulated to be crystallizable to aglass-ceramic having a β-spodumene as the predominant crystalline phaseat a rate of 1-10° C./min to a nucleation temperature (Tn) rangingbetween 700° C. and 810° C.; (ii) maintaining the crystallizable glassat the nucleation temperature to produce a glass article comprisingand/or a nucleated crystallizable glass; (iii) heating the nucleatedcrystallizable glass at a rate of 1-10° C./min to a crystallizationtemperature (Tc) ranging between 850° C. and 1200° C.; (iv) maintainingthe nucleated crystallizable glass at the crystallization temperature toproduce glass-ceramic having a β-spodumene as the predominantcrystalline phase.

In still further aspects, other methods for making a glass-ceramicinclude (i) heating a nucleated crystallizable glass at a rate of 1-10°C./min to a crystallization temperature (Tc) ranging between 850° C. and1200° C.; (ii) selecting the crystallization temperature to tune thecolor based on a correlation between a* and b*, wherein (a) the color ispresented in CIELAB color space coordinates for an observer angle of 10°and a CIE illuminant F02 determined from reflectance spectrameasurements using a spectrophotometer with specular reflectanceincluded comprising (1) CIE a* in a range from about −0.5 to about 0.5;(2) CIE b* in a range from about −2.5 to about 2; and (3) CIE L* in arange from about 90 to about 93; and (b) based on the correlationbetween a* and b*, a plot of a* vs b* has a slope of delta b*/delta a*between about 8 and about 22; and (iii) maintaining the nucleatedcrystallizable glass at the crystallization temperature to produce aglass-ceramic having β-spodumene as the predominant crystalline phase.

The article comprising and/or white, opaque, β-spodumene glass-ceramics,IXable, white, opaque, β-spodumene glass-ceramics, and/or IX, white,opaque, β-spodumene glass-ceramics might be used in a variety ofelectronic devices or portable computing devices, which might beconfigured for wireless communication, such as, computers and computeraccessories, such as, “mice”, keyboards, monitors (e.g., liquid crystaldisplay (LCD), which might be any of cold cathode fluorescent lights(CCFLs-backlit LCD), light emitting diode (LED-backlit LCD) . . . etc,plasma display panel (PDP) . . . and the like), game controllers,tablets, thumb drives, external drives, whiteboards . . . etc.; personaldigital assistants (PDAs); portable navigation device (PNDs); portableinventory devices (PIDs); entertainment devices and/or centers, devicesand/or center accessories such as, tuners, media players (e.g., record,cassette, disc, solid-state . . . etc.), cable and/or satellitereceivers, keyboards, monitors (e.g., liquid crystal display (LCD),which might be any of cold cathode fluorescent lights (CCFLs-backlitLCD), light emitting diode (LED-backlit LCD) . . . etc, plasma displaypanel (PDP) . . . and the like), game controllers . . . etc.; electronicreader devices or e-readers; mobile or smart phones . . . etc. Asalternative examples, white, opaque, β-spodumene glass-ceramics, IXable,white, opaque, β-spodumene glass-ceramics, and/or IX, white, opaque,β-spodumene glass-ceramics might be used in automotive, appliances, andeven architectural applications.

Numerous other aspects of embodiments, features, and advantages of thisdisclosure will appear from the following description and theaccompanying drawings. In the description and/or the accompanyingdrawings, reference is made to exemplary aspects and/or embodiments ofthis disclosure which can be applied individually or combined in any waywith each other. Such aspects and/or embodiments do not represent thefull scope of this disclosure. Reference should therefore be made to theclaims herein for interpreting the full scope of this disclosure. In theinterest of brevity and conciseness, any ranges of values set forth inthis specification contemplate all values within the range and are to beconstrued as support for claims reciting any sub-ranges having endpointswhich are real number values within the specified range in question. Byway of a hypothetical illustrative example, a recitation in thisdisclosure of a range of from about 1 to 5 shall be considered tosupport claims to any of the following ranges: 1-5; 1-4; 1-3; 1-2; 2-5;2-4; 2-3; 3-5; 3-4; and 4-5. Also in the interest of brevity andconciseness, it is to be understood that such terms as “is,” “are,”“includes,” “having,” “comprises,” and the like are words of convenienceand are not to be construed as limiting terms and yet may encompass theterms “comprises,” “consists essentially of,” “consists of,” and thelike as is appropriate.

These and other aspects, advantages, and salient features of thisdisclosure will become apparent from the following description, theaccompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a plot of the color coordinates a* vs. b*;

FIG. 2 shows a plot of the crystallization temperature vs. colorcoordinate L*;

FIG. 3 shows a plot of Fe₂O₃ concentration vs color coordinates a*, b*,and L*;

FIG. 4 shows a plot of a* vs. b* as a function of crystallizationtemperature;

FIG. 5 shows a plot of b* vs L* as a function of crystallizationtemperature;

FIG. 6 shows a plot of % total reflectance over a range of wavelengths;and

FIG. 7 shows a plot of the difference in the color coordinates a*, b*,and L* before and after ion-exchanging a glass-ceramic.

DETAILED DESCRIPTION

In the following description of exemplary aspects and/or embodiments ofthis disclosure, reference is made to the accompanying drawings thatform a part hereof, and in which are shown by way of illustrationspecific aspects and/or embodiments in which this disclosure may bepracticed. While these aspects and/or embodiments are described insufficient detail to enable those skilled in the art to practice thisdisclosure, it will nevertheless be understood that no limitation of thescope of this disclosure is thereby intended. Alterations and furthermodifications of the features illustrated herein, and additionalapplications of the principles illustrated herein, which would occur toone skilled in the relevant art and having possession of thisdisclosure, are to be considered within the scope of this disclosure.Specifically, other aspects and/or embodiments may be utilized, logicalchanges (e.g., without limitation, any one or more of chemical,compositional {e.g., without limitation, any one or more of chemicals,materials, . . . and the like}, electrical, electrochemical,electromechanical, electro-optical, mechanical, optical, physical,physiochemical, . . . and the like) and other changes may be madewithout departing from the spirit or scope of this disclosure.Accordingly, the following description is not to be taken in a limitingsense and the scope of aspects and/or embodiments of this disclosure aredefined by the appended claims. It is also understood that terms such as“top,” “bottom,” “outward,” “inward,” . . . and the like are words ofconvenience and are not to be construed as limiting terms. Also, unlessotherwise specified herein, a range of values includes both the upperand lower limits of the range. For example, a range of between about1-10 mole % includes the values of 1 mole % and 10 mole %. In addition,as used herein, the term “about” modifying a number means within 10% ofthe reported numerical value.

As noted, various aspects and/or embodiments of this disclosure relateto an article and/or machine or equipment formed from and/or includingone or more of white, opaque, β-spodumene glass-ceramics; IXable, white,opaque, β-spodumene glass-ceramics; and/or IX, white, opaque,β-spodumene glass-ceramics of this disclosure. As one example, white,opaque, β-spodumene glass-ceramics; IXable, white, opaque, β-spodumeneglass-ceramics; and/or IX, white, opaque, β-spodumene glass-ceramicsmight be used in a variety of electronic devices or portable computingdevices, which might be configured for wireless communication, such as,computers and computer accessories, such as, “mice”, keyboards, monitors(e.g., liquid crystal display (LCD), which might be any of cold cathodefluorescent lights (CCFLs-backlit LCD), light emitting diode(LED-backlit LCD) . . . etc, plasma display panel (PDP) . . . and thelike), game controllers, tablets, thumb drives, external drives,whiteboards . . . etc.; personal digital assistants (PDAs); portablenavigation device (PNDs); portable inventory devices (PIDs);entertainment devices and/or centers, devices and/or center accessoriessuch as, tuners, media players (e.g., record, cassette, disc,solid-state . . . etc.), cable and/or satellite receivers, keyboards,monitors (e.g., liquid crystal display (LCD), which might be any of coldcathode fluorescent lights (CCFLs-backlit LCD), light emitting diode(LED-backlit LCD) . . . etc, plasma display panel (PDP) . . . and thelike), game controllers . . . etc.; electronic reader devices ore-readers; mobile or smart phones . . . etc. As alternative examples,white, opaque, β-spodumene glass-ceramics; IXable, white, opaque,β-spodumene glass-ceramics; and/or IX, white, opaque, β-spodumeneglass-ceramics might be used in automotive, appliances, and evenarchitectural applications. To that end, it is desirable thatcrystallizable glasses thereto are formulated to have a sufficiently lowsoftening point and/or a sufficiently low coefficient of thermalexpansion so as to be compatible with manipulation into complex shapes.

White, opaque, β-spodumene glass-ceramics having β-spodumene as thepredominant crystalline phase and crystallizable glasses formulated tobe crystallized to white, opaque, β-spodumene glass-ceramics havingβ-spodumene as the predominant crystalline phase according to aspectsand/or embodiments of this disclosure, in mole %, include: SiO₂ in arange from about 62 to about 75; Al₂O₃ in a range from about 10 to about18; Li₂O in a range from about 5 to about 14; B₂O₃ in a range from about2 to about 12; and Fe₂O₃ in a range from about 0.4 to about 2, while inalternative aspects, in mole % Fe₂O₃ is in a range from about 0.5 toabout 2; about 0.6 to about 2, about 0.7 to about 2, about 0.8 to about2, about 0.9 to about 2, about 1 to about 2, about 1.5 to about 2, about0.4 to about 1.5, about 0.5 to about 1.5, about 0.6 to about 1.5, about1 to about 2 about, or about 1 to about 1.5. In alternative aspects, thewhite, opaque, β-spodumene glass-ceramics having β-spodumene as thepredominant crystalline phase and crystallizable glasses formulated tobe crystallized to white, opaque, β-spodumene glass-ceramics havingβ-spodumene as the predominant crystalline phase according to aspectsand/or embodiments of this disclosure, in mole %, can also include MgOin a range from 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ ina range from about 2 to about 5; Na₂O in a range from 0 to about 5; K₂Oin a range from 0 to about 4; and SnO₂ in a range from about 0.05 toabout 0.5.

In other alternative aspects, white, opaque, β-spodumene glass-ceramicshaving β-spodumene as the predominant crystalline phase andcrystallizable glasses formulated to be crystallized to white, opaque,β-spodumene glass-ceramics having β-spodumene as the predominantcrystalline phase according to aspects and/or embodiments of thisdisclosure, in mole %, include: SiO₂ in a range from about 62 to about75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a range fromabout 5 to about 14; B₂O₃ in a range from about 2 to about 12; and ametal oxide selected from group consisting of CoO, Cr₂O₃, Cu₂O, Sb₂O₃,In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof in a range fromabout 0.01 to about 2. In alternative aspects, the white, opaque,β-spodumene glass-ceramics having β-spodumene as the predominantcrystalline phase and crystallizable glasses formulated to becrystallized to white, opaque, β-spodumene glass-ceramics havingβ-spodumene as the predominant crystalline phase according to aspectsand/or embodiments of this disclosure, in mole %, can also include: MgOin a range from 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ ina range from about 2 to about 5; Na₂O in a range from 0 to about 5; K₂Oin a range from 0 to about 4; and SnO₂ in a range from about 0.05 toabout 0.5. It can be difficult to modify the color coordinates of white,β-spodumene glass-ceramics having β-spodumene as the predominantcrystalline phase because color and opacity are controlled by the rutilecrystallite phase. However, including transition metal oxides, such asthe ones listed above, (e.g., Fe₂O₃, CoO, Cr₂O₃, Cu₂O, MnO₂, Sb₂O₃,In₂O₃, Bi₂O₃, NiO, V₂O₃, and Ta₂O₅) in white, β-spodumene glass-ceramicsexpands the color tunability.

In some aspects, such glass-ceramics and crystallizable glasses exhibitthe following compositional criteria:

the ratio of:

${the}\mspace{14mu} {mole}\mspace{14mu} {sum}{\mspace{11mu} \;}{total}\mspace{14mu} {{of}\mspace{14mu}\lbrack \underset{\_}{{{Li}_{2}O} + {{Na}_{2}O} + {K_{2}O} + {MgO} + {ZnO}} \rbrack}\mspace{14mu} {to}\mspace{14mu} {the}\mspace{14mu} {moles}\mspace{14mu} {{of}\mspace{14mu}\lbrack {{Al}_{2}O_{3}} \rbrack}$

can be in a range from about 0.8 to about 1.5.

By formulating crystallizable glasses to have prespecified values ofthis ratio, it is possible to maximize the B-spodumene in theglass-ceramics made using such crystallizable glasses.

In some additional aspects, such glass-ceramics exhibit the followingcrystal phase assemblage:

-   -   (1) β-spodumene solid solutions exhibiting Li₂O:Al₂O₃:SiO₂        ratios ranging from 1:1:4.5-1:1:8 or, alternatively, from        1:1:4.5-1:1:7, and comprising at least 70 wt % of the        crystalline phase;    -   (2) at least one Ti-containing crystalline phase comprising:        -   a. about 2.5-8 wt % of the crystalline phases of the            glass-ceramic,        -   b. an acicular morphology exhibiting a length ≧about 50 nm,            and        -   c. rutile; and optionally,    -   (3) One or more crystalline phases exhibiting a spinel structure        and comprising 1-10 wt % of the crystalline phase.

In further aspects, such glass-ceramics exhibit opaqueness and/or anopacity ≧85% for a 0.8 mm thickness over the wavelength range of 400nm-700 nm.

In still further aspects, when measurement results obtained betweenabout 350 nm-800 nm are presented in CIELAB color space coordinates fora CIE illuminant F02, such glass-ceramics exhibit a level of lightness(L*) above about 90, in some alternative aspects, L* ranging from about90 to about 93. In additional aspects and again presenting the resultsin CIELAB color space coordinates for a CIE illuminant F02, such glassceramic articles exhibit a* values ranging from about −0.5 to 0.5 and b*values ranging from about −2.5 to about 2.

As noted, glass-ceramics according to aspects and/or embodiments of thisdisclosure exhibiting or having “β-spodumene solid solution as thepredominant crystalline phase” (alternatively stated “β-spodumene ss asthe predominant crystalline phase” or “β-spodumene as the predominantcrystalline phase”) means that β-spodumene solid solution (alternativelystated “β-spodumene ss” or “β-spodumene”) constitutes greater than about70 percent by weight (wt %) of the all the crystalline phases of aglass-ceramic according to aspects and/or embodiments of thisdisclosure. Non limiting examples of other possible crystalline phasesof glass-ceramics according to aspects and/or embodiments of thisdisclosure include: β-quartz solid solutions (“β-quartz ss” or“β-quartz”); β-eucryptite solid solutions (“β-eucryptite ss” or“β-eucryptite”); spinel solid solutions (“spinel ss” or “spinel” {suchas e.g., gahnite . . . etc.}); Ti containing crystalline phases (such ase.g., rutile, anatase, magnesium titanates {such as e.g., karrooite(MgTi₂O₅) . . . etc.}, aluminum titanates {such as e.g., tielite(Al₂TiO₅) . . . etc.}, . . . etc.); cordierites (such as e.g.,{Mg,Fe}₂Al₃{Si₅AlO₁₈} to {Fe,Mg}₂Al₃{Si₅AlO₁₈}), and the like.

A predominance of β-spodumene solid solution in β-spodumeneglass-ceramics according to aspects and/or embodiments of thisdisclosure can be beneficial when such glass-ceramics are subjected toone or more IX treatments to produce IX glass-ceramics. For example, thestructure of β-spodumene can exhibit flexibility without a breakdown ofthe framework when Li ions are exchanged for a variety of cations (e.g.,ions of Na, K, Rb . . . etc.).

According to some aspects and/or embodiments of this disclosure,β-spodumene glass-ceramics can be characterized as being opaque and/orbeing white. In such cases, Applicants have found that to achievedesired opacity and/or desired whiteness levels such β-spodumeneglass-ceramics include one or more Ti-containing crystalline phases,which include rutile, as a minor crystalline phase. Example of such oneor more Ti-containing crystalline phases include any of rutile (TiO₂)and, optionally, can include one or more of anatase (TiO₂), karrooite(MgTi₂O₅), tielite (Al₂TiO₅) . . . etc., and mixtures thereof. When itis desirable to achieve a desired opacity and desired whiteness levels,Applicants have found that to achieve a desired degree of opacity andwhiteness such β-spodumene glass-ceramics include one or moreTi-containing crystalline phases, which include rutile, can be acicularcrystals exhibiting in some aspects a length ≧49 nm, in other aspects alength ≧110 nm, and in still other aspects a length ≧1 μm, while in someinstances up to 2 μm.

Spinels are crystalline oxides having the generic formula AB₂O₄ and thebasic spinel structure that is cubic. The prototype spinel structure isthat of magnesium aluminate (MgAl₂O₄). In the basic spinel structure,O-atoms fill the sites of a face centered cubic (FCC) array; A-atomsoccupy some of tetrahedral sites (A-sites) in the FCC structure; andB-atoms occupy octahedral sites (B-sites) in the FCC structure. In thenormal spinels, the A and B atoms are different, A is a +2 ion and B isa +3 ion. In disordered spinels the +2 ions and +3 ions are randomlydistributed over the A-sites and B-sites. In inverse spinels the A-sitesare occupied by +3 ions with the consequence that the B-sites have anequal mixture of +2 ions and +3 ions and the A and B atoms can be thesame. In some instances some A-sites can be occupied by +2 ions, +3ions, and/or +4 ions while in the same or other instances B-sites can beoccupied by +2 ions, +3 ions, and/or +4 ions. Some examples of A-atomsinclude zinc, nickel, manganese, magnesium, iron, copper, cobalt . . .etc. Also some examples of B-atoms include aluminum, antimony, chromium,iron, manganese, titanium, vanadium . . . etc. A wide range of solidsolutions are common in spinels and can be represented by the genericformula (A_(x) ¹A_(1-x) ²)[B_(y) ¹B_(2-y) ²]O₄. For example, completesolid solution is obtained between ZnAl₂O₄ and MgAl₂O₄, which canrepresented by the formula (Zn_(x) ¹Mg_(1-x) ²)Al₂O₄. According to someaspects and/or embodiments of this disclosure, β-spodumeneglass-ceramics include one or more crystalline phases exhibiting aspinel structure, which in aspects have compositions close to that ofgahnite, ZnAl₂O₄. Also it has been found that as the amounts of ZnO orZnO and Al₂O₃ are increased, such β-spodumene glass-ceramics can haveincreased amounts of gahnite. The refractive index (n) of gahnite canrange between 1.79-1.82, which can be higher than that of β-spodumene(n=between 1.53-1.57) but significantly less than that of rutile(n=between 2.61-2.89). Also, in contrast to β-spodumene and rutile thatare tetragonal, being cubic spinels can exhibit no birefringence.Therefore, Applicants believe that spinels in general and Zn-containingspinels in particular would have less of an influence on color ofβ-spodumene glass-ceramics than would rutile.

In aspects of embodiments of this disclosure when β-spodumeneglass-ceramics include Ti-containing crystalline phases comprisingrutile, it can range between 2.5 wt % to 6 wt % of the crystallinephases. Applicants have found that by maintaining rutile as at least 2.5wt % of the crystalline phases minimum desired opacity levels can beensured while by maintaining rutile as 6 wt % or less of the crystallinephases desired opacity levels can be maintained while at the same timedesired white levels can be ensured. Stated differently, the TiO₂content of β-spodumene glass-ceramics can range between 2-5 mole % andby maintaining at least 2 mole % minimum desired opacity levels can beensured while by maintaining 5 mole % or less desired opacity levels canbe maintained while at the same time desired white levels can beensured.

For comparison, the refraction index (n) in descending order for severalmaterials follows: rutile (n=between 2.61-2.89); anatase (n=between2.48-2.56); diamond (n=between 2.41-2.43); gahnite (n=between1.79-1.82); sapphire (n=between 1.75-1.78); cordierite (n=between1.52-1.58); β-spodumene (n=between 1.53-1.57); and residual glass(n=between 1.45-1.49). Also for comparison, the birefringence (Δn) indescending order for the some of the same materials follows: rutile(Δn=between 0.25-0.29); anatase (Δn=0.073); sapphire (Δn=0.008);cordierite (Δn=between 0.005-0.017); diamond (Δn=0); and gahnite (Δn=0).Based on the above data, it can be seen that some of the Ti-containingcrystalline phases, and rutile in particular, are among the materialsexhibiting some of the highest refractive indices. In addition, anotherit can be seen that the some of the Ti-containing crystalline phases,and rutile in particular, their relatively high birefringence (Δn), aresult of the anisotropic character of their tetragonal crystalstructure. As a difference in either refractive index or birefringenceincreases among a predominant crystalline phase (e.g., β-spodumene{n=between 1.53-1.57}) and/or any residual glass (n=between 1.45-1.49)and any minor crystalline phases of glass-ceramics scattering of visiblewavelengths can increase in turn increasing opacity. A difference ineach characteristic alone can be beneficial while a difference in botheven more be even more so. Given the differences in both among some ofthe Ti-containing crystalline phases, and rutile in particular, and thebase phase(s) (β-spodumene and any residual glass), the β-spodumeneglass-ceramics of the present disclosure exhibit desirable levelscattering that can be relatively high and, thus the requisite anddesired opacity that likewise can be high.

Al₂O₃ contributes to the β-spodumene glass-ceramics of the presentdisclosure exhibiting β-spodumene as the predominant crystalline phase.As such, a minimum of about 10 mole % Al₂O₃ is desired. Above about 18mole % Al₂O₃ is undesirable as the resultant mullite liquidus makes itdifficult to melt and form crystallizable glasses.

Including Na₂O and K₂O can reduce the melting temperature of thecrystallizable glasses and/or enable shorter crystallization cycles.

Crystallizable glasses and/or β-spodumene glass-ceramics of the presentdisclosure contain 2-12 mole % B₂O₃. Crystallizable glasses of presentdisclosure typically can be melted at a temperature below 1600° C., incertain aspect and/or embodiments below about 1580° C. while in certainother aspect and/or embodiments below about 1550° C., making it possibleto melt in a relatively small commercial glass tank. The inclusion ofB₂O₃ is conducive to the low melting temperature. Further, the additionof B₂O₃ can improve the damage resistance of the glass-ceramics. Forexample, the addition of B₂O₃ can increase a Vickers crack initiationthreshold.

MgO and ZnO can act as fluxes for crystallizable glasses. As such, aminimum mole % sum [MgO+ZnO] of 2 mole % is desired to obtain a glassmelting temperature below 1600° C. Ions of Mg and, to a lesser extent,ions of Zn can participate in the β-spodumene of the β-spodumeneglass-ceramics or can react with Al₂O₃ to form a spinel crystallinephase.

Maintaining Li₂O between 5-14 mole % in crystallizable glassesencourages the formation of β-spodumene solid solution crystallinephases. Also, Li₂O acts as a flux to decrease the melting point of thecrystallizable glasses. As such, a minimum of 5 mole % Li₂O is desiredin order to obtain the desired β-spodumene phase. Above 14 mole % Li₂Ocan be undesirable as unwelcome phases, such as, lithium silicates . . .etc., might result during the formation of glass-ceramics.

An appropriate types and amount of one or more nucleation agents isincluded in crystallizable glasses to facilitate nucleation and/orgrowth of at least β-spodumene as the predominant crystalline phase andany desired one or more minor crystalline phases during the nucleationand/or crystallization heat treatments. Among appropriate types of oneor more nucleation agents are TiO₂, ZrO₂ . . . etc. while amongappropriate amounts are TiO₂: up to 6 mole %; and/or ZrO₂: up to 2 mole% . . . etc. Small amount of SnO₂ appear to enter the rutile phase insolid solution and, as such, might contribute to nucleation. In aspectsand/or embodiments, applicants have found that an inclusion of TiO₂ as anucleation agent is desirable when the formation of one or moreTi-containing phases is desired to achieve a prescribed degree ofopacity and whiteness levels. In other aspects and/or embodiments, aninclusion of ZrO₂ as a nucleation agent can increase nucleationefficiency. Thus, types and amount of one or more nucleation agents iscarefully prescribed. It is noted that in certain aspect and/orembodiments relating to β-spodumene glass-ceramics (optionallyexhibiting β-quartz solid solution), a minimum mole % sum [TiO₂+SnO₂] inexcess of 2.5 mole % is desired as an ingredient of crystallizableglasses. In other words, effective amounts of this mole % sum[TiO₂+SnO₂] are formulated as an ingredient of crystallizable glasses sothat nucleation in an effective manner occurs and growth is achieved toa preselected and appropriate crystal phase assemblage. It is noted thatabove 6 mole % TiO₂ is undesirable as the resultant high rutile liquidushas the potential of increasing difficulties during shape forming ofcrystallizable glasses. Also, it is noted that an inclusion of SnO₂, inaddition to its possible minor contribution to nucleation, can partiallyfunction as a fining agent during a manufacture of crystallizableglasses to contribute to their quality and integrity.

Maintaining the ratio:

$\frac{\lbrack {{TiO}_{2} + {SnO}_{3}} \rbrack}{\lbrack {{SiO}_{2} + {B_{2}O_{3}}} \rbrack}$

in some aspects greater than 0.04 and, in some alternative aspects,greater than 0.05 can contribute to achieving preselected andappropriate crystal phase assemblages that, in turn, contributes toachieving prescribed degrees of opacity and/or whiteness levels.

Also in β-spodumene glass-ceramics and/or their crystallizable glassesaccording to aspect and/or embodiments, applicant have found thatβ-spodumene crystalline phases exhibiting a Li₂O:Al₂O₃:nSiO₂ ratiobetween 1:1:4.5-1:1:8 to be desirable. As such, a minimum ratio of1:1:4.5 is desired to avoid the formation of excessive levels of theunstable residual glass in the resultant β-spodumene glass-ceramics.Above a ratio of 1:1:8 is undesirable as issues with that meltability ofcrystallizable glasses can arise. In some embodiments, MgO can besubstituted for Li₂O such that moles of MgO divided by the mole sum of[MgO+Li2O] can be up to about 30%.

Other properties that can be exhibited by β-spodumene glass-ceramicsaccording to aspects and/or embodiments of this disclosure include oneor more of:

(1) radio and microwave frequency transparency, as defined by a losstangent of less than 0.03 and at a frequency range of between 15 MHz to3.0 GHz;

(2) a fracture toughness greater than 0.8 MPa·m^(1/2);

(3) a Modulus of Rupture (MOR) greater than 20,000 psi;

(4) a Knoop hardness of at least 400 kg/mm²;

(5) a thermal conductivity of less than 4 W/m° C.; and

(6) a porosity of less than 0.1 vol %.

In aspects and/or embodiments relating to articles in general andelectronic device housings or enclosures in particular (each partiallyor completely comprised of β-spodumene glass-ceramics), such articlesand/or β-spodumene glass-ceramics exhibit radio and microwave frequencytransparency, as defined in some aspects by a loss tangent of less than0.02; in alternative aspects of less than 0.01; and in still furtheraspects of less 0.005, the loss tangent determined over a frequencyranging from 15 MHz to 3.0 GHz at about 25° C. This radio and microwavefrequency transparency feature can be especially beneficial for wirelesshand held devices that include antennas internal to the enclosure. Thisradio and microwave transparency allows the wireless signals to passthrough the housings or enclosures and in some cases enhances thesetransmissions. Additional benefits can be realized when such articlesand/or β-spodumene glass-ceramics exhibit a dielectric constantdetermined over a frequency ranging from 15 MHz to 3.0 GHz at about 25°C. of less than about 10; alternatively, less than about 8; or thenagain, less than about 7 in combination with the above values of losstangent.

In still further aspects and/or embodiments of this disclosure relatingto β-spodumene glass-ceramics that have been chemically strengthened,such IX, β-spodumene glass-ceramics exhibit a fracture toughness ofgreater than 0.8 MPa·m^(1/2); alternatively, greater than 0.85MPa·m^(1/2); or then again, greater than 1 MPa·m^(1/2). Independent ofor in combination with the stated fracture toughnesses, such IX,β-spodumene glass-ceramics exhibit a MOR of greater than 40,000 psi or,alternatively, greater than greater than 50,000 psi.

Other aspects and/or embodiments of this disclosure relate to methodsfor forming crystallizable glasses formulated to be crystallizable toglass-ceramics and methods for forming glass-ceramics having aβ-spodumene as the predominant crystalline phase. In aspects, somemethods included melting a mixture of raw materials formulated toproduce upon melting crystallizable glasses including, in mole %, SiO₂in a range from about 62 to about 75; Al₂O₃ in a range from about 10 toabout 18; Li₂O in a range from about 5 to about 14; B₂O₃ in a range fromabout 2 to about 12; and Fe₂O₃ in a range from about 0.4 to about 2,while in alternative aspects, in mole %, including: SiO₂ in a range fromabout 62 to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂Oin a range from about 5 to about 14; B₂O₃ in a range from about 2 toabout 12; MgO in a range from 0 to about 8; ZnO in a range from 0 toabout 4; TiO₂ in a range from about 2 to about 5; Na₂O in a range from 0to about 5; K₂O in a range from 0 to about 4; SnO₂ in a range from about0.05 to about 0.5; and Fe₂O₃ in a range from about 0.4 to about 2, whilein further alternative aspects, in mole %, including: SiO₂ in a rangefrom about 62 to about 75; Al₂O₃ in a range from about 10 to about 18;Li₂O in a range from about 5 to about 14; B₂O₃ in a range from about 2to about 12; and a metal oxide selected from group consisting of CoO,Cr₂O₃, Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinationsthereof in a range from about 0.01 to about 2, while in still furtheralternative aspects, in mole %, including: SiO₂ in a range from about 62to about 75; Al₂O₃ in a range from about 10 to about 18; Li₂O in a rangefrom about 5 to about 14; B₂O₃ in a range from about 2 to about 12; MgOin a range from 0 to about 8; ZnO in a range from 0 to about 4; TiO₂ ina range from about 2 to about 5; Na₂O in a range from 0 to about 5; K₂Oin a range from 0 to about 4; SnO₂ in a range from about 0.05 to about0.5; and a metal oxide selected from group consisting of CoO, Cr₂O₃,Cu₂O, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof ina range from about 0.01 to about 2.

In additional aspects, such mixture of raw materials is formulated toproduce upon melting crystallizable glasses exhibiting the followingcompositional criteria:

the ratio of

$\frac{{the}\mspace{14mu} {mole}\mspace{14mu} {sum}\mspace{14mu} {total}\mspace{14mu} {{of}\mspace{14mu}\lbrack {{{Li}_{2}O} + {{Na}_{2}O} + {K_{2}O} + {MgO} + {ZnO}} \rbrack}\mspace{14mu} {to}}{{moles}\mspace{14mu} {{of}\mspace{14mu}\lbrack {{Al}_{2}O_{3}} \rbrack}}$

can be in a range from about 0.8 to about 1.5.

In still other aspects, such mixture of raw materials is formulated toproduce the above crystallizable glasses upon fining and homogenizationmolten glass compositions at a temperature below about 1600° C. Stillyet other aspects included forming molten crystallizable glasses into aglass article.

In further aspects, some other methods include methods for formingglass-ceramics having a β-spodumene as the predominant crystalline phaseby transforming crystallizable glasses. Such other methods including (i)heating a glass article comprising and/or a crystallizable glassformulated to be crystallizable to a glass-ceramic having a β-spodumeneas the predominant crystalline phase at a rate in a range from about 1°C./min to about 10° C./min to a nucleation temperature (Tn) in a rangefrom about 700° C. to about 810° C.; (ii) maintaining the glass articlecomprising and/or the crystallizable glass at the nucleation temperaturefor a time, for example in a range between ¼ h to 2 h, to produce aglass article comprising and/or a nucleated crystallizable glass; (iii)heating the glass article comprising and/or a nucleated crystallizableglass at a rate in a range from about 1° C./min to about 10° C./min to acrystallization temperature (Tc) in a range from about 850° C. to about1200° C.; (iv) maintaining the glass article comprising and/or thenucleated crystallizable glass at the crystallization temperature for atime, for example in a range between about ¼ h to 4 h, to produce anarticle comprising and/or a glass-ceramic having a β-spodumene as thepredominant crystalline phase; and (v) cooling the article comprisingand/or β-spodumene glass-ceramic to room temperature.

As noted above, in some aspects the glass-ceramic articles exhibitCIELAB color space coordinates of L* ranging from about 90 to about 93,a* values ranging from about −0.5 to about 0.5, and/or b* values rangingfrom about −2.5 to about 2. In some aspects, there can exist acorrelation between a* and b* as a function of crystallizationtemperature, wherein a plot of a* vs. b* has a slope of delta b*/deltaa* between about 8 and about 22 and/or a correlation between L* and b*exists, as a function of crystallization temperature, wherein a plot ofb* vs. L* has a slope delta L*/delta b* of between about 0.75 and about1.25. Also, in some aspects, the crystallization temperature (Tc) can beselected to tune the color based on the correlation between a* and b*and/or a correlation between L* and b*

Anhydrous boric acid may be used as the source of B₂O₃. Spodumene, finealumina, and Al-metaphosphate may be used as the starting materials. Oneskilled in the art can calculate the amount of batch materials usedaccording to the projected final composition of the glass-ceramic. Asmentioned above, a fining agent that has been found to be beneficial isSnO₂ in an amount between about 0.05-0.15 mole %.

The mixed batch materials are then charged into a glass tank and meltedaccording to conventional glass melting process. One skilled in theglass melting art can adjust the composition of the batch within theabove described compositional range to fine tune the melting ease of theglass in order to accommodate the operating capacity and temperature ofthe glass melting tank. The molten glass can be homogenized and finedusing conventional methods. Whilst some glasses having a meltingtemperature over 1600° C. can crystallize to form β-quartz and/orβ-spodumene solid solution glass-ceramic, such high temperature meltingusually has to be carried out in expensive melting tanks with specialdesign. In addition, the liquidus behavior of such high meltingtemperature glass usually requires higher temperature pressing andmolding.

The homogenized, fined and thermally uniform molten glass is then formedinto desired shapes. Various shaping may be used, such as casting,molding, pressing, rolling, floating, and the like. Generally, the glassshould be delivered at a viscosity lower than the liquidus viscosity(hence a temperature higher than the liquidus temperature). Takepressing for example. The glass is first delivered to high temperaturemolds and formed into glass articles with desired shape, surface textureand surface roughness by using a plunger. To obtain low surfaceroughness and a precise surface contour, precision plungers are requiredto press the glass gobs filled in the molds. It is also required thatthe plungers will not introduce IR absorbing oxides or other defectsonto the surface of the glass article should high IR transmission isrequired. The moldings are then removed from the molds and transferredto a glass annealer to remove enough stress in the moldings for furtherprocessing where necessary and desirable. Thereafter, the cooled glassmoldings are inspected, analyzed of chemical and physical properties forquality control purpose. Surface roughness and contour may be tested forcompliance with product specification.

To produce the glass-ceramic article of the present disclosure, the thusprepared glass articles are placed into a crystallization kiln toundergo the crystallization process. The temperature-temporal profile ofthe kiln is desirably program-controlled and optimized to ensure thatthe glass moldings and other glass articles, such as glass plates andthe like, are formed into glass-ceramic articles having β-spodumene asthe predominant crystalline phase. As described above, the glasscomposition and the thermal history during the crystallization processdetermine the final crystalline phases, their assemblage and crystallitesizes in the final product. Generally, the glass articles are firstheated to a nucleation temperature (Tn) range where crystal nuclei startto form. Subsequently, they are heated to an even higher maximumcrystallization temperature Tc to obtain β-spodumene crystallization. Itis often desired to keep the articles at Tc for a period of time so thatcrystallization reaches a desired extent. In order to obtain theglass-ceramic articles of the present disclosure, the nucleationtemperature Tn is between 700-810° C., and the maximum crystallizationtemperature Tc is between 850° C.-1200° C. After crystallization, thearticles are allowed to exit the crystallization kiln and are cooled toroom temperature. One skilled in the art can adjust Tn, Tc and thetemperature-temporal profile of the crystallization cycle to accommodatethe different glass compositions within the above-described range. Theglass-ceramic article of the present disclosure can advantageouslyexhibit an opaque white coloring.

The glass-ceramic article of the present disclosure may be furtherprocessed before its final intended use. One such post-processingincludes IX treatment of the glass-ceramic to form an IX glass-ceramicarticle having at least a portion of at least one surface subjected toan IX process, such that the IX portion of the least one surfaceexhibits a compressive layer having a depth of layer (DOL) greater thanor equal to 2% of the overall article thickness while exhibiting acompressive stress (σ_(s)) in the surface of at least 300 MPa. Any IXprocess known to those in the art might be suitable as long as the aboveDOL and compressive stress (σ_(s)) are achievable.

In a more particular embodiment the housing or enclosure exhibits anoverall thickness of 2 mm and compressive layer exhibiting a DOL of 40μm with that compressive layer exhibiting a compressive stress (σ_(s))of at least 150 MPa. Again any IX process which achieves these featuresis suitable.

It is noted that in addition to single step IX processes, multiple IXprocedures might be utilized to create a designed IX profile forenhanced performance. That is, a stress profile created to a selecteddepth by using IX baths formulated with differing concentration of ionsor by using multiple IX baths formulated using different ion specieshaving different ionic radii.

As used herein, the term “ion exchanged” is understood to mean treatingthe heated β-spodumene glass-ceramic or crystallizable glass compositionwith a heated solution containing ions having a different ionic radiusthan ions that are present in the glass-ceramic surface, crystallizableglass, and/or bulk, thus replacing those ions with smaller ions with thelarger ions or vice versa depending on the ion exchange (“IX”)temperature conditions. Potassium (K) ions, for example, could eitherreplace, or be replaced by, sodium (Na) ions in the glass-ceramic, againdepending upon the IX temperature conditions. Alternatively, otheralkali metal ions having larger atomic radii, such as (Rb) rubidium orcesium (Cs) could replace smaller alkali metal ions in the glass-ceramicor crystallizable glass. Similarly, other alkali metal salts such as,but not limited to, sulfates, halides, and the like may be used in theion exchange (“IX”) process.

In the instant method, it is contemplated that both types of IX can takeplace; i.e., larger for smaller ions are replaced and/or smaller forlarger ions are replaced. In one some aspects and/or embodiments, themethod involves IX (particularly lithium-for-sodium ion exchange) theβ-spodumene glass-ceramic or crystallizable glass by placing it in anNaNO₃ bath at temperatures between 310-490° C. for times up to 20 h. Insome embodiments, the ion-exchange under the above conditions canachieve a DOL of at least 80 microns. In other aspects and/orembodiments, the IX can be accomplished utilizing mixed potassium/sodiumbaths at similar temperatures and times; e.g., an 80/20 KNO₃/NaNO₃ bathor alternatively a 60/40 KNO₃/NaNO₃ at comparable temperatures. In stillother aspects and/or embodiments, a two-step IX process is contemplatedwherein the first step is accomplished in a Li-containing salt bath;e.g. the molten salt bath can be a high temperature sulfate salt bathcomposed of Li₂SO₄ as a major ingredient, but diluted with Na₂SO₄, K₂SO₄or Cs₂SO₄ in sufficient concentration to create a molten bath. This IXstep functions to replace the larger sodium ions in the β-spodumeneglass-ceramic with the smaller lithium ions which are found in theLi-containing salt bath. The second IX step functions to exchange Nainto the β-spodumene glass-ceramic and can be accomplished as above by aNaNO₃ bath at temperatures between 310° C. and 490° C.

Characterization of Crystallizable Glasses, Glass-ceramics, IXableGlass-ceramics, and/or IX Glass-ceramics

CIELAB color space coordinates (e.g., CIE L*; CIE a*; and CIE b*; or CIEL*, a*, and b*; or L*, a*, and b*) for describing the color ofβ-spodumene glass-ceramics; IXable, β-spodumene glass-ceramic; and/orIX, β-spodumene glass-ceramics according to aspects and/or embodimentsof this disclosure were determined by methods known to those in the artfrom total reflectance—specular included—measurements, such as, thosedescribed by F. W. Billmeyer, Jr., “Current American Practice in ColorMeasurement,” Applied Optics, Vol. 8, No. 4, pp. 737-750 (April 1969),which are incorporated by reference herein. Namely, totalreflectance—specular included—measurements were made of surfacesprepared to an optical polish using sample disks measuring about 33mmØ×0.8 mm thick. Equipment and supplies for making such totalreflectance—specular included—measurements and translating results toobtain L*; a*; and b* color space coordinates included:

an ultraviolet-visible-near infrared (UV-VIS-NIR) spectrophotometerequipped with integrating sphere such as the commercially availableVarian Cary 5G or PerkinElmer Lambda 950 UV-VIS-NIR spectrophotometersappropriately equipped and configured so as to be enabled for totalreflectance—specular included—measurements in the wavelength range250-3300 nm (e.g., ultraviolet (UV: 300-400 nm), visible (Vis: 400-700nm), and infrared (IR: 700-2500 nm); and

an analytical software (UV/VIS/NIR application pack of the GRAMSspectroscopy software suite commercially available from ThermoScientific West Palm Beach, Fla., US) for color measurements coupled toa UV-VIS-NIR spectrophotometer that translated measurement results tothe CIELAB color space coordinates (L*; a*; and b*) based on F02illuminant and a 10-degree standard observer.

Identity of the crystalline phases of crystal phase assemblages and/orcrystal sizes of a crystalline phase for crystallizable glasses,glass-ceramics, IXable glass-ceramics, and/or IX glass-ceramicsaccording to aspects and/or embodiments of this disclosure weredetermined by X-ray diffraction (XRD) analysis techniques known to thosein the art, using such commercially available equipment as the model asa PW1830 (Cu Kα radiation) diffractometer manufactured by Philips,Netherlands. Spectra were typically acquired for 20 from 5 to 80degrees.

Elemental profiles measured for characterizing surfaces ofcrystallizable glasses, glass-ceramics, IXable glass-ceramics, and/or IXglass-ceramics according to aspects and/or embodiments of thisdisclosure were determined by analytical techniques know to those in theart, such as, electron microprobe (EMP); x-ray photoluminescencespectroscopy (XPS); secondary ion mass spectroscopy (SIMS) . . . etc.

Compressive stress (σ_(s)) in a surface layer can be determined bymethods described in Sglavo, V. M. et al., “Procedure for residualstress profile determination by curvature measurements,” Mechanics ofMaterials 37:887-898 (2005), which is incorporated by reference hereinin its entirety.

Flexural Strength of crystallizable glasses, glass-ceramics, IXableglass-ceramics, and/or IX glass-ceramics according to aspects and/orembodiments of this disclosure can be characterized by methods known tothose in the art, such as, those described in ASTM C1499 (and itsprogeny, all herein incorporated by reference) “Determination ofMonotonic Equibiaxial Flexural Strength Advanced Ceramics,” ASTMInternational, Conshohocken, Pa., US.

Young's Modulus, Shear Modulus, and Poisson's Ratio of crystallizableglasses, glass-ceramics, IXable glass-ceramics, and/or IX glass-ceramicsaccording to aspects and/or embodiments of this disclosure can becharacterized by methods known to those in the art, such as, thosedescribed in ASTM C1259 (and its progeny, all herein incorporated byreference) “Standard Test Method for Dynamic Young's Modulus, ShearModulus, and Poisson's Ratio for Advanced Ceramics by Impulse Excitationof Vibration,” ASTM International, Conshohocken, Pa., US.

Knoop hardness of crystallizable glasses, glass-ceramics, IXableglass-ceramics, and/or IX glass-ceramics according to aspects and/orembodiments of this disclosure can be characterized by methods known tothose in the art, such as, those described in ASTM C1326 (and itsprogeny, all herein incorporated by reference) “Standard Test Methodsfor Vickers Indentation Hardness of Advanced Ceramics,” ASTMInternational, Conshohocken, Pa., US.

Vickers hardness of crystallizable glasses, glass-ceramics, IXableglass-ceramics, and/or IX glass-ceramics according to aspects and/orembodiments of this disclosure can be characterized by methods known tothose in the art, such as, those described in ASTM C1327 (and itsprogeny, all herein incorporated by reference) Standard Test Methods forVickers Indentation Hardness of Advanced Ceramics,” ASTM International,Conshohocken, Pa., US.

Vickers indentation cracking threshold measurements of crystallizableglasses, glass-ceramics, IXable glass-ceramics and/or IX glass-ceramicsaccording aspects and/or embodiments of this disclosure can becharacterized by methods known to those in the art, such as by applyingand then removing an indentation load to a Vickers indenter as describedin ASTM C1327 to the surface of the material to be tested at a rate of0.2 mm/min. The maximum indentation load is held for 10 seconds. Theindentation cracking threshold is defined at the indentation load atwhich 50% of 10 indents exhibit any number of radial/median cracksemanating from the corners of the indent impression. The maximum load isincreased until the threshold is met for a given glass or glass-ceramiccomposition. All indentation measurements are performed at roomtemperature in 50% relative humidity.

Coefficient of thermal expansion (CTE) of crystallizable glasses,glass-ceramics, IXable glass-ceramics, and/or IX glass-ceramicsaccording to aspects and/or embodiments of this disclosure can becharacterized by methods known to those in the art, such as, thosedescribed in ASTM E228 (and its progeny, all herein incorporated byreference) Standard Test Method for Linear Thermal Expansion of SolidMaterials with a Push-Rod Dilatometer,” ASTM International,Conshohocken, Pa., US.

Fracture toughness (K_(1C)) of crystallizable glasses, glass-ceramics,IXable glass-ceramics, and/or IX glass-ceramics according to aspectsand/or embodiments of this disclosure can be characterized by methodsknown to those in the art, such as, those described in ASTM C1421 (andits progeny, all herein incorporated by reference) Standard Test Methodsfor Determination of Fracture Toughness of Advanced Ceramics at AmbientTemperature,” ASTM International, Conshohocken, Pa., US and/or usingchevron notched short bar (CNSB) specimens and/or methods substantiallyaccording to ASTM E1304 C1421 (and its progeny, all herein incorporatedby reference) “Standard Test Method for Plane-Strain (Chevron-Notch)Fracture Toughness of Metallic Materials,” ASTM International,Conshohocken, Pa., US.

EXAMPLES

The following examples illustrate the advantages and features of thisdisclosure and in are no way intended to limit this disclosure thereto.

Inasmuch as the sum of the individual constituents totals or veryclosely approximates 100, for all practical purposes the reported valuesmay be deemed to represent mole %. The actual crystallizable glass batchingredients may comprise any materials, either oxides, or othercompounds, which, when melted together with the other batch components,will be converted into the desired oxide in the proper proportions.

The crystallizable glasses listed in Table I were used in the followingexamples. The amount of Fe₂O₃ varies among compositions 1-12 toillustrate the effects of Fe₂O₃ on the color coordinates. Composition 1also has a B₂O₃ concentration below 2 mol % and serves as a control forcomparison of compositions with 2 mol % or greater of B₂O₃.

Example 1

Crystallizable glass compositions 1-12 were formed by heating thecompositions to a nucleation temperature of 780° C. and maintaining thecompositions at the nucleation temperature for 2 hours. Then each ofcompositions 1-12 were heated to a crystallization temperature of either900° C., 925° C., or 950° C. at a rate of 5° C./min and held at thecrystallization temperature for 4 hours and allowed to cool. The CIELABcolor coordinates L*, a*, and b* for an observer angle of 10° and a CIEilluminant F02 were determined for each composition using reflectancespectra measurements using a spectrophotometer with specular reflectanceas described above.

The L*, a*, and b* values indicate that the color of glass-ceramiccompositions can be tuned based on Fe₂O₃. As shown in FIG. 1, a plot ofa* vs. b* for each composition illustrates that b* increases as theconcentration of Fe₂O₃ increases. Also, as shown in FIG. 2, a plot ofcrystallization temperature vs. L* illustrates that L* generallyincreases with crystallization temperature and that L* decreases with adecrease in the concentration of Fe₂O₃. FIG. 3 further illustrates thetunability of glass-ceramics based on increasing the Fe₂O₃concentration. FIG. 3 illustrates that a* varies with Fe₂O₃concentration according to the following relationship: a*=17.726[molconc Fe₂O₃]²+4.847[mol conc Fe₂O₃]−0.2822; b* varies with Fe₂O₃concentration according to the following relationship: b*=6.5303 [molconc Fe₂O₃]−0.8066; and L* varies with Fe₂O₃ concentration according tothe following relationship: L*=−19.135[mol conc Fe₂O₃]+94.435. Therelationship for a* has an R² fit value of 0.9121; the relationship forb* has an R² fit value of 0.9813; and the relationship of r L* has a R²fit value of 0.8202.

Example 2

Compositions 1 and 7 were heated at a rate of 5° C./min from thenucleation temperature of 780° C. to a crystallization temperatures of850° C., 900° C., 950° C., 1,000° C., and 1,050° C. The compositionswere held at the crystallization temperatures for 4 hours. The CIELABcolor coordinates L*, a*, and b* for an observer angle of 10° and a CIEilluminant F02 were determined for each composition using reflectancespectra measurements using a spectrophotometer with specular reflectanceas described above. FIG. 4 illustrates a plot of a* vs. b* as a functionof increasing crystallization temperature for composition 1 andcomposition 7. The function for a* vs. b* for composition 1, which has aB₂O₃ of less than 2 mole %, is b*=6.6579a*+1.7341. This relationship hasan R² fit value of 0.9934. The function for a* vs. b* for composition 7,is b*=20.65a*+7.1318. This relationship has an R² fit value of 0.994.FIG. 5 illustrates a plot of b* vs. L* as a function of increasingcrystallization temperature for composition 1 and composition 7. Thefunction for b* vs. L* for composition 1, which has a B₂O₃ of less than2 mole % and 0 mole % Fe₂O₃, is L*=0.991b*+93.669. This relationship hasan R² fit value of 0.9722. The function for b* vs. L* for composition 7,is L*=1.0389b*+9.749. This relationship has an R² fit value of 0.9742.

Example 3

Compositions 1-7 were heated at a nucleation temperature of 780° C. for2 hours, heated at a rate of 5° C./min to a crystallization temperatureof 950° C., and then heated at 950° C. for four hours. The totalreflectance of each composition was measured over a range ofwavelengths. FIG. 6 is a plot of reflectance (%) vs. wavelength (nm).This plot illustrates that the reflectance decreases with an increase inFe₂O₃. Despite the absorption, the glass-ceramics obtained bycrystallizing the example compositions at a temperature ranging from900° C. to 950° C. have a color coordinate L* over 90, which is anindication of their brightness.

Example 4

Compositions 2-7 were heated at a nucleation temperature of 780° C. for2 hours, heated at a rate of 5° C./min to a crystallization temperatureof 950° C., and then heated at 950° C. for four hours. The CIELAB colorcoordinates L*, a*, and b* for an observer angle of 10° and a CIEilluminant F02 were determined for each composition using reflectancespectra measurements using a spectrophotometer with specular reflectanceas described above. Then, the samples were ion exchanged in a NaNO₃ bathat 430° C. for one hour. The CIELAB color coordinates L*, a*, and b* foran observer angle of 10° and a CIE illuminant F02 were determined againfor each sample after the ion-exchange process. FIG. 7 shows the changein L*, a*, and b* before and after the ion-exchange process for eachsample. The small changes in L*, a*, and b* indicate that keeping theratio of the mole sum total of Li₂O+Na₂O+K₂O+MgO+ZnO to the moles ofAl₂O₃ in a range from about 1 to about 1.5 a stable color is maintainedbefore and after ion exchanging.

Various modifications and variations can be made to the materials,methods, and articles described herein. Other aspects of the materials,methods, and articles described herein will be apparent fromconsideration of the specification and practice of the materials,methods, and articles disclosed herein. It is intended that thespecification and examples be considered as exemplary.

We claim:
 1. A glass-ceramic article comprising: a composition, in mole%, comprising: i) SiO₂ in a range from about 62 to about 75, ii) Al₂O₃in a range from about 10 to about 18, iii) Li₂O in a range from about 5to about 14, iv) B₂O₃ in a range from about 2 to about 12, and v) Fe₂O₃in a range from about 0.4 to about 2, wherein β-spodumene is thepredominant crystalline phase.
 2. The glass-ceramic article of claim 1,wherein the glass-ceramic article comprises a color presented in CIELABcolor space coordinates for an observer angle of 10° and a CIEilluminant F02 determined from reflectance spectra measurements using aspectrophotometer with specular reflectance included comprising: a) CIEa* in a range from about −0.5 to about 0.5; b) CIE b* in a range fromabout −2.5 to about 2; and c) CIE L* in a range from about 90 to about93.
 3. The glass-ceramic article of claim 1, wherein the compositioncomprises Fe₂O₃ in a range from about 0.5 to about 2 mole %.
 4. Theglass-ceramic article of claim 1, further comprising a compressivestress layer formed by ion-exchange.
 5. The glass-ceramic article ofclaim 4, wherein the compressive stress layer has a compressive stressof at least 300 MPa and a depth of layer (DOL) of at least about 2% ofan overall thickness of the article.
 6. The glass-ceramic article ofclaim 1, wherein the composition further comprises: vi) MgO in a rangefrom 0 to about 8, vii) ZnO in a range from 0 to about 4, viii) TiO₂ ina range from about 2 to about 5, ix) Na₂O in a range from 0 to about 5,x) K₂O in a range from 0 to about 4, and xi) SnO₂ in a range from about0.05 to about 0.5, and wherein the glass-ceramic article furthercomprises: a ratio of the mole sum total [Li₂O+Na₂O+K₂O+MgO+ZnO]/molesof [Al₂O₃] in a range from about 0.8 to about 1.5.
 7. A process formaking a glass-ceramic comprising: a) heating a crystallizable glass ata rate in a range from about 1° C./min to about 10° C./min to anucleation temperature in a range from about 700° C. and 810° C.,wherein, in mole %, the crystallizable glass comprises: i) SiO₂ in arange from about 62 to about 75, ii) Al₂O₃ in a range from about 10 toabout 18, iii) Li₂O in a range from about 5 to about 14, iv) B₂O₃ in arange from about 2 to about 12, and v) Fe₂O₃ in a range from about 0.4to about 2, and b) maintaining the crystallizable glass at thenucleation temperature to produce a nucleated crystallizable glass; c)heating the nucleated crystallizable glass at a rate in a range fromabout 1° C./min to about 10° C./min to a crystallization temperature ofin a range from about 850° C. to about 1200° C.; and d) maintaining thenucleated crystallizable glass at the crystallization temperature toproduce a glass-ceramic having β-spodumene as a predominant crystallinephase.
 8. The process of claim 7, wherein the glass-ceramic articlecomprises a color presented in CIELAB color space coordinates for anobserver angle of 10° and a CIE illuminant F02 determined fromreflectance spectra measurements using a spectrophotometer with specularreflectance included comprising: a) CIE a* in a range from about −0.5 toabout 0.5; b) CIE b* in a range from about −2.5 to about 2; and c) CIEL* in a range from about 90 to about
 93. 9. The process of claim 8,wherein a correlation between a* and b* exists, as a function ofcrystallization temperature, wherein a plot of a* vs. b* has a slope ofdelta b*/delta a* between about 8 and about
 22. 10. The process of claim9, wherein a correlation between b* and L* exists, as a function ofcrystallization temperature, wherein a plot of b* vs. L* has a slope ofdelta L*/delta b* of between about 0.75 and about 1.25.
 11. The processof claim 9, further comprising selecting the crystallization temperatureto tune the color based on the correlation between a* and b*.
 12. Theprocess of claim 10, further comprising selecting the crystallizationtemperature to tune the color based on the correlation between a* and b*and the correlation between b* and L*.
 13. The process of claim 7,wherein the crystallizable glass further comprises: vi) MgO in a rangefrom 0 to about 8, vii) ZnO in a range from 0 to about 4, viii) TiO₂ ina range from about 2 to about 5, ix) Na₂O in a range from 0 to about 5,x) K₂O in a range from 0 to about 4, and xi) SnO₂ in a range from about0.05 to about 0.5, and xii) a ratio of the mole sum total[Li₂O+Na₂O+K₂O+MgO+ZnO]/moles of [Al₂O₃] in a range from about 0.8 toabout 1.5
 14. A process for making a glass-ceramic comprising: a)heating a nucleated crystallizable glass at a rate in a range from about1° C./min to about 10° C./min to a crystallization temperature of in arange from about 850° C. to about 1200° C., wherein, in mole %, thenucleated crystallizable glass comprises: i) SiO₂ in a range from about62 to about 75, ii) Al₂O₃ in a range from about 10 to about 18, iii)Li₂O in a range from about 5 to about 14, iv) B₂O₃ in a range from about2 to about 12, v) Fe₂O₃ in a range from about 0.4 to about 2, and b)selecting the crystallization temperature to tune the color based on acorrelation between a* and b*, wherein: i) the color is presented inCIELAB color space coordinates for an observer angle of 10° and a CIEilluminant F02 determined from reflectance spectra measurements using aspectrophotometer with specular reflectance included comprising: (1) CIEa* in a range from about −0.5 to about 0.5; (2) CIE b* in a range fromabout −2.5 to about 2; and (3) CIE L* in a range from about 90 to about93; and ii) based on the correlation between a* and b*, as a function ofcrystallization temperature, a plot of a* vs. b* has a slope of deltab*/delta a* of between about 8 and about 22; and maintaining thenucleated crystallizable glass at the crystallization temperature toproduce a glass-ceramic having β-spodumene as a predominant crystallinephase.
 15. The process of claim 14, wherein based on a correlationbetween b* and L*, as a function of crystallization temperature, a plotof b* vs. L* has a slope delta L*/delta b* of between about 0.75 andabout 1.25.
 16. The process of claim 15, further comprising selectingthe crystallization temperature to tune the color based on thecorrelation between a* and b* and the correlation between b* and L*. 17.The process of claim 14, further comprising ion exchanging the glassceramic to form a compressive stress layer.
 18. The process of claim 17,wherein the compressive stress layer has a compressive stress of atleast 300 MPa and a depth of layer (DOL) of at least about 2% of anoverall thickness of the glass-ceramic.
 19. The process of claim 14,wherein the crystallizable glass further comprises: vi) MgO in a rangefrom 0 to about 8, vii) ZnO in a range from 0 to about 4, viii) TiO₂ ina range from about 2 to about 5, ix) Na₂O in a range from 0 to about 5,x) K₂O in a range from 0 to about 4, and xi) SnO₂ in a range from about0.05 to about 0.5, and xii) a ratio of the mole sum total[Li₂O+Na₂O+K₂O+MgO+ZnO]/moles of [Al₂O₃] in a range from about 0.8 toabout 1.5.
 20. A glass-ceramic article comprising: a) a composition, inmole %, comprising: i) SiO₂ in a range from about 62 to about 75, ii)Al₂O₃ in a range from about 10 to about 18, iii) Li₂O in a range fromabout 5 to about 14, iv) B₂O₃ in a range from about 2 to about 12, andv) a metal oxide selected from group consisting of CoO, Cr₂O₃, Cu₂O,MnO₂, Sb₂O₃, In₂O₃, Bi₂O₃, NiO, V₂O₃, Ta₂O₅, and combinations thereof ina range from about 0.01 to about 2; wherein β-spodumene is thepredominant crystalline phase.
 21. The glass-ceramic article of claim20, wherein the glass-ceramic article comprises a color presented inCIELAB color space coordinates for an observer angle of 10° and a CIEilluminant F02 determined from reflectance spectra measurements using aspectrophotometer with specular reflectance included comprising: a) CIEa* in a range from about −0.5 to about 0.5; b) CIE b* in a range fromabout −2.5 to about 2; and c) CIE L* in a range from about 90 to about93.
 22. The glass-ceramic article of claim 20, further comprising acompressive stress layer formed by ion-exchange.
 23. The glass-ceramicarticle of claim 22, wherein the compressive stress layer has acompressive stress of at least 300 MPa and a depth of layer (DOL) of atleast about 2% of an overall thickness of the article.
 24. Theglass-ceramic article of claim 20, wherein the composition furthercomprises: vi) MgO in a range from 0 to about 8, vii) ZnO in a rangefrom 0 to about 4, viii) TiO₂ in a range from about 2 to about 5, ix)Na₂O in a range from 0 to about 5, x) K₂O in a range from 0 to about 4,and xi) SnO₂ in a range from about 0.05 to about 0.5, and wherein theglass-ceramic article further comprises: a ratio of the mole sum total[Li₂O+Na₂O+K₂O+MgO+ZnO]/moles of [Al₂O₃] in a range from about 0.8 toabout 1.5.